1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus, and, more particularly, a sheet processing apparatus and an image forming apparatus which fold and bind a sheet bundle.
2. Description of the Related Art
Conventionally, there are image forming apparatuses such as copying machines or laser beam printers which have a sheet processing apparatus which takes in sheets after images are formed on the sheets, and folds and binds the taken sheets or a sheet bundle. To bind a sheet bundle, these conventional sheet processing apparatuses, for example, form a sheet bundle in a booklet by overlaying a predetermined number of sheets equal to or less than about 20 sheets and folding the sheets by means of a seaming/folding machine of a folding unit. In addition, the sheet bundle folded by this seaming/binding machine includes a sheet bundle which is simply folded, a sheet bundle which is saddle-stitched and folded, or a sheet bundle which is folded by an adhesive (perfect binding) instead of binding a sheet bundle with a thread or staple.
However, any sheet bundle has some elasticity, and therefore, as illustrated in FIGS. 8A and 8B which will be described below, the sheet bundle is folded and then the periphery of a spine which is a folded portion of the sheet bundle swells, thereby forming a U shape. This sheet bundle cannot lie flat and, when the sheets are stacked, the sheet bundle becomes unstable and is likely to collapse. Therefore it is difficult to store or carry a sheet bundle by stacking sheets.
Hence, to prevent this problem, a sheet processing apparatus has a pressing roller which presses the spine of a sheet bundle run along the spine while pressing the spine, and crushes the curved spine and squares the spine in a square shape. This sheet processing apparatus has a nipping unit which nips adjacent portions of the spine from both of front and back surfaces of the sheet bundle, and a pressing unit (pressing roller) which presses the spine projecting outward from the nipping unit from the direction orthogonal to both of the front and back surfaces and squares the spine. Further, when the spine is squared, the spine is squared by moving the nipping unit and the pressing unit integrally along the spine (U.S. Pat. No. 7,431,274).
However, when this conventional sheet processing apparatus presses the spine while moving the pressing unit along the spine, not only the pressed spine but also other spine of the sheet bundle to be pressed next is distorted depending on rigidity or the extent of projection of the sheet bundle. When this pressing unit moves along the spine of the sheet bundle in this state, the spine to be pressed next is also pressed in a distorted state.
Further, when the spine to be pressed next is pressed in the distorted state in this way, there is a problem that the shape after a squaring process does not become stable and how the shape looks is not good, for example, a wrinkle is made. In addition, it is difficult to stabilize the amount of distortion or how the sheet bundle is distorted when the spine is pressed, and, further, when distortions before pressing are different, how the sheet bundle is pressed changes.
The present invention is made in view of the above-described problem, and the present invention provides a sheet processing apparatus and an image forming apparatus which can stabilize the shape after a spine squaring process.